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International audience; Hybrid van der Waals heterostructures made of 2D materials and organic molecules exploit the high sensitivity of 2D materials to all interfacial modifications and the inherent versatility of the organic compounds. In this study, we are interested in the quinoidal zwitterion/MoS 2 hybrid system in which organic crystals are grown by epitaxy on the MoS 2 surface and reorganize in another polymorph after thermal annealing. By means of field-effect transistor measurements recorded in situ all along the process, atomic force microscopy and density functional theory calculations we demonstrate that the charge transfer between quinoidal zwitterions and MoS 2 strongly depends on the conformation of the molecular film. Remarkably, both the field effect mobility and the current modulation depth of the transistors remain unchanged which opens up promising prospects for efficient devices based on this hybrid system. We also show that MoS 2 transistors enable fast and accurate detection of structural modifications that occur during phases transitions of the organic layer. This work highlights that MoS 2 transistors are remarkable tools for on-chip detection of molecular events occurring at the nanoscale, which paves the way for the investigation of other dynamical systems.

We present a combined scanning tunneling microscopy (STM), noncontact atomic force microscopy (nc-AFM), and differential reflectance spectroscopy (DRS) study aiming at the characterization of the relationship between structural and optical properties of a supramolecular assembly of a curcuminoid derivative adsorbed on two kinds of substrates: bilayers of KCl on Au(111) [2 ML KCl/Au(111)] and KCl(001) bulk. The molecule features a difluoroboron (${\mathrm{BF}}_{2}$) complex as well as cyano (CN) end groups, which induces an internal dipole moment of a few Debyes. The comparative study of the structural properties of the molecules adsorbed on 2 ML KCl/Au(111), which allows for STM characterization, and KCl bulk, which allows for nc-AFM and DRS characterization, shows that the molecular adsorption is similar on both substrates. STM and nc-AFM give evidence that the on-surface condensation of the molecules into supramolecular assemblies is steered by two main interactions: the molecule-substrate interaction between the CN end groups of the molecule and the cationic ${\mathrm{K}}^{+}$ species, as well as an in-plane intermolecular interaction involving ${\mathrm{BF}}_{2}$ and phenyl groups. Based on density functional theory calculations, we propose an epitaxial molecular structure described by a two-molecule unit cell forming an $8\ifmmode\times\else\texttimes\fi{}2$ supercell on the KCl surface that grows in form of ribbons along the $\ensuremath{\langle}100\ensuremath{\rangle}$ or $\ensuremath{\langle}010\ensuremath{\rangle}$ directions of the substrate, which are polar. Among several possible stereoisomers for the molecule, it is found that the balance between vertical and in-plane interactions selects the one whose size matches the distance between ${\mathrm{K}}^{+}$ species to condense on the substrate and not the most stable one stemming from the gas phase. Finally, optical spectroscopy in solvent and DRS measurements performed on the supramolecular assembly exhibit similar features identified as three absorption peaks tracing a $\ensuremath{\pi}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{*}$ electronic transition followed by a vibronic progression, plus a broader band featuring a set of higher-energy transitions. Due to both the weak electrostatic interactions and a lack of $\ensuremath{\pi}\text{\ensuremath{-}}\ensuremath{\pi}$ interactions in the condensed molecular phase, DR spectra exhibit no major excitonic effect, but a nonrigid redshift of the molecular absorption band compared to the situation in solvent. This observation is found to be consistent with the isomer selection due to the transition from the gas phase to the on-surface condensed phase.

International audience; Dihydrotetraazapentacene (DHTAP) molecules can be dehydrogenated on the surface to form tetraazapentacene (TAP), by applying a high electric field between the tip of a scanning tunnelling microscope (STM) and a metallic substrate in the zero-current limit. The method can be applied either to single molecules or more extended layers by successively scanning a selected area using an STM tip. Scheme 1 Dehydrogenation of DHTAP to TAP using an electric field E.

We used indirect nanoplasmonic sensing (INPS) coupled with mass spectrometry to study CO and oxygen adsorption as well as CO oxidation, on Pt nanoparticles, in the Torr pressure range. Due to an optimization of the physical parameters of our plasmonic sample, we obtain a highly sensitive probe that can detect gas adsorption of a few hundredths of a monolayer, even with a very low number density of Pt particles. Moreover and for the first time, a similarity is observed between the sign and the evolution of the localized surface plasmon resonance (LSPR) peak shift and the work function measurements for CO and oxygen chemisorption. Controlling the size, shape, and surface density of Pt particles, the turnover frequency (TOF) has also been accurately determined. For similar experimental conditions, the TOF is close to those measured on Pt/oxide powder catalysts and Pt(100) single crystals.

The invention relates to a process for preparing single-crystal thin films of pure metals or alloys, by deposition and growth of grains on a (0001) sapphire having a miscut comprised between 0.2° and 5°, and oriented towards a specific plane. It also relates to a single-crystal thin film obtainable by such a process and uses thereof., L'invention concerne un procédé de préparation de films minces monocristallins de métaux ou d'alliages purs, par dépôt et croissance de grains sur un saphir (0001) présentant une mauvaise découpe comprise entre 0,2° et 5°, et orientés vers un plan spécifique. L'invention concerne également un film mince monocristallin pouvant être obtenu par un tel procédé et ses utilisations.

International audience; In this study, thin films of pure and chromium doped TiO 2 were deposited by RF magnetron sputtering technique with recourse to a low cost powder target. The dopant concentration was varied from 0 to 6 wt %. The structural, optical, electrical and photocatalytic properties of the sputtered deposited films were systematically investigated on the basis of the incorporated Cr content. XRD analysis revealed that the elaborated films are polycrystalline and have a preferential orientation along the (110) plane, characteristic of the rutile phase. The optical measurements showed a good homogeneity of the films and a high transmission that reached 85% in the visible range. The high transparency of these films allows their use as an optical window in thin film solar cells. Optical band gap decreased from 3.44 eV to 3.32 eV with the increase of the Cr content. In addition, the impedance spectroscopy analysis showed that the conductivity increases along with increasing frequency and temperature, and that the conduction mechanism follows the correlated barrier hopping model. The test of the catalytic efficiency of the chromiumdoped TiO 2 thin films at different concentrations demonstrated a degradation of the methylene blue (MB) and an improvement of the photocatalytic activity and that the 6 wt% doped thin films have the best photocatalytic activity.

International audience; Hybrid organic/inorganic interfaces could pave the way to chemically designed or new multifunctional electronic devices, in particular in the spintronics field where, for instance, the interfacial spin polarization can be tuned through chemical interactions and surface modifications. We report herein, for the very first time, the assays of self-assembled monolayers (SAMs) formation on Mn 5 Ge 3 surface. Interestingly, Mn 5 Ge 3 is a ferromagnetic metal possessing interesting features for spintronics such as a high Curie temperature, a capability to grow epitaxially on Ge germanium that is a key point for integration in the mainstream Si technology. Mn 5 Ge 3 thin films are synthesized by molecular beam epitaxy under ultra-vacuum condition. We studied the Mn 5 Ge 3 surface prior to deposition of SAMs, meaning the surface exposed to air and different solvents, by XPS and contact angle measurements leading to the value of the surface tension of this surface. Then SAMs of octanethiol and perfluorodecanethiol are formed on Mn 5 Ge 3 surface at room temperature. The best experimental conditions to form the SAMs are found for an immersion time of 36 h and a concentration of 4 mM.

The adsorption of 5,14-dihydro-5,7,12,14-tetraazapentacene (DHTAP) on Cu(110) has been investigated at different temperatures and coverages by means of scanning tunneling microscopy (STM) and low-e...

The real-time formation of Si single layers on Ag(110) and Ag(111) has been investigated at different temperatures, by a combination of scanning tunnelling microscopy, grazing incidence X-ray diffraction, optical differential reflectance spectroscopy and density functional theory. It is shown that the reactivity of the Ag substrate with the Si atoms is important. The formation of Si nanoribbons on Ag(110) is accompanied by a missing-row reconstruction of the Ag surface, and the removed Ag atoms form additional islands. The mechanism of Si island formation on Ag(111) involves the insertion of single Si atoms within the Ag top layer and the exchange with Ag atoms. The released Ag atoms diffuse to steps edges and form new Ag fingers. This strong interaction between Si and Ag atoms with possible alloying, questions the actual structures of the different Si phases observed on Ag(110) and Ag(111), and usually attributed to silicene single layers.

International audience; The combined characterization of structural and optical properties of organic interfaces adsorbed on inorganic solid substrates down to the molecular scale is crucial from a fundamental point of view, but also if one tackles efficient applied devices. In this work, a set of joint structural and optical analyses of self-assemblies of π-conjugated bis-pyrene derivatives upon adsorption on two alkali halides bulk insulators is reported. The structural analysis is performed by means of noncontact atomic force microscopy in ultrahigh vacuum either at room or at the liquid-nitrogen temperature with molecular resolution. The surface coverage ranges from the submonolayer (ML) regime up to 5 ML. In situ optical spectroscopy is performed by means of differential reflectance (DR) spectroscopy. A thorough fitting methodology of the DR spectra allows us to derive the complete dielectric function of the molecular adlayers treated in an anisotropic formalism, albeit restricted to an uniaxial approximation. Conclusions regarding the process of condensation of the molecules into H aggregates from its early stages up to the solid molecular phase are drawn. This work highlights three main reasons to bridge high-resolution structural and optical characterization of the molecular layers, which all point towards the necessity to constrain the fitting process, namely, (i) characterizing the growth mode of the molecules, (ii) identifying the structural order of the resulting assemblies, and (iii) discriminating their constitutive phases by means of molecular resolution imaging.

International audience; We demonstrate in this work that using nanoplasmonic sensing it is possible to follow the adsorption/desorption of water molecules on gold nanodisks nanofabricated by electron beam lithography. This quantitative method is highly sensitive allowing the detection of a few hundredths of adsorbed monolayer. Disk parameters (height, diameter, inter-disk distance) have been optimized after finite-difference time-domain (FDTD) simulations in order to obtain the best localized surface plasmon resonance (LSPR) signal-to-noise ratio. Finally, we have precisely measured the adsorption kinetics of water on gold as a function of the relative humidity of the surrounding medium.

International audience; We demonstrate in this work that the indirect nanoplasmonic sensing lets us follow the adsorption/desorption of water molecules on soot particles that are a major contributor of the global warming. Increasing the relative humidity of the surrounding medium we measure a shift in wavelength of the localized surface plasmon resonance response of gold nanodisks on which soot particles are deposited. We show a singular and reversible blue shift with hydrophilic aircraft soot particles interpreted from a basic model as a reversible morphological change of the soot aggregates. This new method is highly sensitive and interesting to follow the change of optical properties of aerosols during their aging in the atmosphere, where they can adsorb and react with different gas molecules.

International audience; We demonstrate the kinetically controlled growth of one-dimensional Co nanomagnets with a high lateral order on a nanopatterned Ag(110) surface. First, self-organized Si nanoribbons are formed upon submonolayer condensation of Si on the anisotropic Ag(110) surface. Depending on the growth temperature, individual or regular arrays (with a pitch of 2 nm) of Si nanoribbons can be grown. Next, the Si/Ag(110) system is used as a novel one-dimensional Si template to guide the growth of Co dimer nanolines on top of the Si nanoribbons, taking advantage of the fact that the thermally activated process of Co diffusion into the Si layer is efficiently hindered at 220 K. Magnetic characterization of the Co nanolines using X-ray magnetic circular dichroism reveals that the first atomic Co layer directly adsorbed onto the Si nanoribbons presents a weak magnetic response. However, the second Co layer exhibits an enhanced magnetization, strongly suggesting a ferromagnetic ordering with an in-plane easy axis of magnetization, which is perpendicular to the Co nanolines.

The adsorption and growth of 1,4-di-n-octyloxy-2,5-bis(pyren-1-ylethenyl)benzene (bis-pyrene) thermo-evaporated onto a Au(111) single crystal surface has been investigated using low temperature scanning tunneling microscopy (STM) under ultrahigh vacuum (UHV) conditions. At low coverage, bis-pyrene molecules arrange into molecular chains, which exclusively grow on the fcc domains of Au(111) herringbone reconstruction. In these molecular chains, the individual molecules are connected by a van der Waals interaction between the alkyl chains of the molecules. As the coverage is increased the hcp domains of Au(111) herringbone reconstruction are also decorated by molecular chains. Finally, for coverages above 0.5 ML, four distinct ordered commensurate phases are observed, which suggests a delicate balance between the intermolecular and the molecule–surface interactions. A detailed analysis of the STM images suggests that distinct molecular configurations of the bis-pyrene compound in these ordered phases are at...

The synthesis of borondifluoride complexes of curcuminoids and one- and two-photon absorption properties of 1 and 2 are described. These compounds allow the preparation of organic nanoparticles that were characterized using DLS, TEM and AFM revealing sizes around 50–60 nm for 2. The fluorescence emission spectra of the nanoparticles are red-shifted compared to those of the dye diluted in dichloromethane solution reaching the NIR region of the spectrum. This effect stems from the occurrence of π–π interactions in the solid state, as revealed from the X-ray crystal structure analysis of 1. Those dyes exhibit a significant two-photon absorption cross-section in solution and nanoparticle suspension in water. The nanoparticles are shown to behave as NIR-to-NIR fluorescent nanomaterials that could be potentially used for studies in the biological transparency window. Two-photon brightnesses of ca. 13–14 GM are obtained for both dyes, which are amongst some of the highest values reported to date for NIR-to-NIR luminophores.

International audience; Pre- and postintroduction of substituents with respect to the macrocyclization step leads to previously unknown N-substituted azacalixphyrins. The stepwise synthetic approach has been studied in detail to highlight the key role of the N-substituents of the precursors and/or intermediates in terms of reactivity. Based on a combined experimental and theoretical investigation, the relationship between the properties of the macrocycles and their degree of substitution is rationalized. Depending on the nature of the N-substituents, the formation of supramolecular ribbon-like structures could also be observed, as demonstrated by combined TEM, SEM, AFM, and FTIR experiments.

In surface science and model catalysis, cerium oxide (ceria) is mostly grown as an ultra-thin film on a metal substrate in the ultra-high vacuum to understand fundamental mechanisms involved in diverse surface chemistry processes. However, such ultra-thin films do not have the contribution of a bulk ceria underneath, which is currently discussed to have a high impact on in particular surface redox processes. Here, we present a fully oxidized ceria thick film (180 nm) with a perfectly stoichiometric CeO2(111) surface exhibiting exceptionally large, atomically flat terraces. The film is well-suited for ceria model studies as well as a perfect substitute for CeO2 bulk material.

This study presents an innovative synthesis of dihydrotetraazapentacene (DHTAP) and the scanning tunneling microscopy (STM) investigation of the initial stages of its growth on Au(111). We were able to demonstrate that, up to the fourth monolayer, the DHTAP films show a high structural order and growths in perfect epitaxy. This behavior can be unequivocally attributed to the stabilizing effect of intralayer hydrogen bonding interactions.

International audience; We report here in situ measurements of the evolution of the Ag(110) surface during Si growth, using scanning tunneling microscopy and grazing incidence x-ray diffraction. We provide compelling evidence of an Ag(110) surface reconstruction associated with the release of Ag atoms induced by the growth of Si nanoribbons. Our results are in agreement with a missing row reconstruction of the Ag layer underneath the nanoribbons. This challenges the current understanding of the Si growth on nonreconstructed Ag(110), interpreted within the framework of silicene models.

International audience; Silicene, the considered equivalent of graphene for silicon, has been recently synthesized on Ag(111) surfaces. Following the tremendous success of graphene, silicene might further widen the horizon of two-dimensional materials with new allotropes artificially created. Due to stronger spin-orbit coupling, lower group symmetry and different chemistry compared to graphene, silicene presents many new interesting features. Here, we focus on very important aspects of silicene layers on Ag(111): First, we present scanning tunneling microscopy (STM) and non-contact Atomic Force Microscopy (nc-AFM) observations of the major structures of single layer and bi-layer silicene in epitaxy with Ag(111). For the (3 3 3) reconstructed first silicene layer nc-AFM represents the same lateral arrangement of silicene atoms as STM and therefore provides a timely experimental confirmation of the current picture of the atomic silicene structure. Furthermore, both nc-AFM and STM give a unifying interpretation of the second layer (!3 3 !3)R 6 306 structure. Finally, we give support to the conjectured possible existence of less stable, ,2% stressed, (!7 3 !7)R 6 19.16 rotated silicene domains in the first layer.

The Mn5Ge3 compound, thanks to its room-temperature ferromagnetism, metallic character and ability to epitaxially grow on germanium, acts as a potential candidate for spin injection into group-IV semiconductors. Understanding and controlling Ge overgrowth behaviour on Mn5Ge3/Ge heterostructures represents a crucial step to realize Ge/Mn5Ge3/Ge multilayers for numerous spintronic applications. Here, we have combined structural and morphological characterizations with magnetic analyses to study the mechanisms of Ge overgrowth on epitaxial Mn5Ge3 layers in the temperature range of 450-550 degrees C. It is found that deposited Ge instantly reacts with Mn to form a Mn5Ge3 surface layer, which, acting as a surfactant, continuously floats upwards from the growing surface to a distance larger than 70 nm. New Ge layers are successively formed underneath, allowing such a floating Mn5Ge3 surface layer to be stabilized by epitaxy. These observations can be considered as a typical example in which the stabilization of metastable thin films by epitaxy can overcome thermodynamic equilibrium. We have also investigated the effect of carbon adsorption on the top of the Mn5Ge3 layer prior to Ge deposition to control the Mn:Ge reaction. It is shown that adsorbed carbon effectively reduces the out-diffusion of Mn from Mn5Ge3, allowing Ge layers to stack up on top of Mn5Ge3. However, at temperatures of 450-550 degrees C, carbon may react with Mn to form manganese carbides and the resulting Ge overlayers are found to change their orientation from the (111) plane to the (001) plane, which has a higher surface energy. Finally, a strategy to realize Ge/Mn5Ge3/Ge multilayers will be addressed.

Room temperature conductance-elongation curves of gold atomic wires are measured using a Scanning Tunnelling Microscope Break Junction technique. Landauer's conductance plateaus are individually identified and statistically analysed. Both the probabilities to observe, and the lengths of the two last plateaus (at conductance values close to 2e2/h and 4e2/h) are studied. All results converge to show that the occurrence of these two conductance plateaus on a conductance-elongation curve are statistically independent events., 8 pages, 5 figures. Accepted for publication in Nanotechnology

The first stages of the growth of silicon on Ag(0 0 1) at moderate temperatures start by the formation of a p(3 · 3) superstructure, which continuously evolves with increasing coverage toward a more complex superstructure. In this paper, the atomic arrangement of the p(3 · 3) and of the ‘‘complex’’ superstructure has been investigated using scanning tunnelling microscopy, surface X-ray diffraction and low energy electron diffraction. The atomic model retained for the p(3 · 3) reconstruction consists in four silicon atoms (tetramers) adsorbed near hollow and bridge sites of the top most Ag(0 0 1) surface layer. For higher coverages, i.e., when the ‘‘complex’’ superstructure starts to develop, the silicon overlayer forms periodic stripes, most probably bi-layers, with a graphitic like structure. � 2006 Elsevier B.V. All rights reserved.

Various factors that can affect the coverage dependence of diffusivity for the case of surface hetero-diffusion have been analyzed. Simple Monte Carlo modeling of the diffusion process shows that diffusivity can either increase or decrease with coverage, depending on whether the interaction between diffusing atoms is repulsive or attractive. The results of more detailed simulations show that these simple predictions are reasonable. If the diffusing species undergoes first-order phase transitions as a function of coverage, then important perturbations of the diffusion profiles will result. One final factor in coverage dependence of diffusivity is considered, namely, the possible effects of coverage on the surface step structure. An analysis of diffusion profiles obtained for the spreading of Bi over Cu(100) shows that the major effects of coverage on diffusivity come about from the presence of phase transitions, and from the inherent coverage dependence of diffusivity in systems with short-range repulsion.

Scanning tunneling microscopy (STM) has been used to study the various possible structures of adsorbed Bi on the Cu(1 0 0) surface, after equilibration at a temperature of 520 K. All of the structures previously identified by X-ray diffraction (lattice gas, c(2 × 2), c(9√2 × √2) R 45°, and p(10 × 10), in order of increasing Bi-coverage) were found to be present on a single sample produced by diffusing Bi onto the Cu(1 0 0) surface from a 3-d source. By investigating the possible coexistence of various pairs of phases, it was demonstrated that the c(2 × 2) phase transforms to the c(9√2 × √2) R 45° phase by a first order transition, whereas the transition from c(9√2 × √2) R 45° to p(10 × 10) is continuous. In addition, the structure of surface steps was studied as a function of Bi-coverage. The results showed that the presence of Bi changes the nature of the step–step interactions at the Cu(1 0 0) surface from repulsive to attractive. The attractive step–step interactions transform any small deviations from the nominal (1 0 0) orientation of the Cu substrate into (3 1 0) microfacets. When compared with the known equilibrium crystal shape (ECS) of Bi-saturated Cu, the observed microfaceting may imply that the ECS of Cu–Bi alloys is temperature dependent.

We have used reflection electron microscopy to study (1) the drift velocity of positive monolayer islands on Si(111) under near equilibrium conditions and direct current (dc) heating and (2) the difference between the velocity of monoatomic steps heated by direct current in the step-up and step-down direction under free sublimation. Our results obtained at around $1200\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ show that the islands move in the opposite direction of the dc and the step velocity for ascending steps with respect to the dc direction is smaller than the velocity for opposite steps with the same interstep distance. These two experimental results are a clear qualitative prove of the existence of an Ehrlich-Schwoebel effect on Si(111). We analyze the experimental results for the step dynamics within a modified Burton, Cabrera, and Frank model. We find that a quantitative agreement between the model and the experimental results needs an enhanced effect of the electric field on the diffusion species.